Hardware checks
Check the keyboard, mouse, screen and microphone you are on.
Check the machine you are sitting at: every key on the keyboard, both mouse buttons and the wheel, the microphone level and what it sounds like played back, the screen's real size and pixel ratio, a dead-pixel hunt in pure colour. These use the browser's own device APIs and send nothing anywhere.
Mouse TestButtons, wheel, double-click and rateScreen SizeScreen, viewport and pixel ratio at onceKeyboard TesterPress a key and watch it registerCPS TestClicks per second, one to a hundredDead Pixel TestPure full-screen fills, one at a timeMic TestSee the level, then hear it backTone GeneratorAny frequency, four waves, volume guardedWebcam TestSee your camera and what it deliversVirtual PianoPlay piano with your computer keyboardArabic KeyboardType Arabic letters and copy them outReaction Time TestFive trials, median not just your bestOnline RulerA screen ruler that is actually to scale